Total posts : 45366
[quote:8ba329bb39=”RadioheadC”]The ancient part 15 FM standard (circa 1970s) was 100 milliwatts (mw) for FM, but chances are if you are using even close to 50 mw now, you would probably be in violation. Obviously the exact power output to push beyond the standards will depend on antenna, how and where it is installed, feed lines and a whole bunch of other variables.[/quote:8ba329bb39]
How true. Using the equation
Field Strength in dB above 1 microvolt/meter =
104.77 + ERPi – 20 log (D)
ERPi = Effective Radiated Power in dB referenced
to 1kW from an isotropic radiator
D = Distance in kilometers
and applying the necessary units conversions says that input power of ~11.5 nanowatts to a linear 1/2-wave dipole will produce the FCC’s maximum field of 250 uV/m at a distance of three meters. A nanowatt is a millionth of a milliwatt! Physically small transmit antennas for VHF are MUCH more efficient than for MW.
BTW, for equal conditions this equation produces the same numbers that the FCC “Curves” program does for free space conditions.
If you want to experiment with a PC based, legal Part 15 FM station you might look for a “LineX” FM tx that plugs into, and is powered by a computer USB port. It also takes its stereo audio feed from the USB port, and transmits it on one of several PLL-selected freqs in the FM band. I’ve used one, and it sounds pretty good.